This invention relates to a frequency-modulation(FM) modulator in a cellular wireless telephone, more particularly to an FM modulator which modulates corresponding frequencies after linearly compensating for modulation characteristics on a respective one of a plurality of channels.
In general, frequency characteristics in modulation of a cellular wireless telephone are not shown linearly over all frequency channels because the cellular wireless telephone employs a large number of, for example, hundreds or thousands of frequency channels.
FIG. 1 shows a schematic construction of one of known FM modulators. The FM modulator has a varactor diode 1 for frequency deviation of an input signal from an input channel, a resonator 3 for resonating the deviated frequency from the varactor diode 1 to provide therefrom a resonance frequency, and a voltage controlled oscillator 4 for generating an oscillation frequency depending on the resonance frequency provided from the resonator 3.
FIG. 2 shows a voltage-to-capacitance characteristic diagram of the varactor diode 1, and FIG. 3 shows a waveform of input channel signal in a voltage-to-time representation.
A capacitance of the varactor diode 1 varies as an input voltage (shown in FIG. 2). Therefore, if a signal shown in FIG. 3 is input through an input node P1, respective input voltages according to time, for example +Va, +Vb, -Vc, and -Vd, are supplied to the varactor diode 1, so that the capacitance of the varactor diode 1 varies to Ca, Cb, Cc, and Cd, respectively. The varied value of the capacitance varies the resonance frequency of the resonator 3 composed of the coil L1 and the capacitor C1. The resonance frequency f.sub.o of the resonator 3 is determined by the following equation. ##EQU1## wherein, f.sub.o is resonance frequency, L is inductance, and C is capacitance.
The resonance frequency f.sub.o of the resonator 3 provides an oscillation frequency to the voltage controlled oscillator 4. The conventional FM modulator of FIG. 1, as described above, has uniform frequency modulation characteristics due to the linear properties of the varactor diode in a small number of channels with a narrow frequency band, but has a problem that inconsistent frequency modulation characteristics due to the unlinear properties of the varactor diode in the multichannel with wide frequency band causes an error.